Graphene and graphene quantum dots have seen numerous applications as useful materials, including applications in optics, electronics, and biomedical applications. Graphene, a typical sp2 carbon-based material, has attracted tremendous interests after its first being isolated in 2004. Graphene oxides and graphene quantum dots have excellent physical and chemical properties, graphene and its derivatives, such as graphene oxide (GO) and graphene quantum dots (GQDs), have exhibited great potential in optical, electrochemical, and biomedical applications. Among them, biomedical applications of GQDs represent a relatively new but fast growing area, for example, bioimaging, biosensor materials, and drug delivery. By combining the advantages of graphene and quantum dots (QDs), these graphene quantum dots are expected to be a promising therapeutic and diagnostic tool for various diseases.
Although several methods have been developed for the synthesis of graphene quantum dots, there are still many deficiencies in those methods such as the need for high energy or radiation times. The starting material of those methods is graphene oxide which needs to be synthesized from graphite first and those methods take long time and multiple steps to obtain graphene quantum dots with low yield. Methods of preparing quantum dots have been attempted using vapor deposition processes, such as MOCVD (Metal Organic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy). Chinese Application No. 2013-10200476 described methods of making graphene quantum dots by first grinding a flake graphite to produce graphite nanoparticles, oxidizing the graphite nanoparticles with nitric acid at low temperature for 20 hours, dried, and then heated to 450° C. Furthermore, the methods described previously do not produce graphene quantum dots with high yields and with monodisperse and narrow size distributions.
Therefore, there remains a need for improved methods of making graphene quantum dots, especially small graphene quantum dots with monodisperse size distributions.